Why Is My Molly So Fat? Is It Actually Pregnant?

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The most prevalent type of fish in my aquarium is probably the molly fish. That is primarily because of its beauty and hardy characteristics. However, more than once, I have noticed that my molly fish become fat and bloated. That got me worried. To determine whether they are sick or merely pregnant, I began to investigate the issue pretty extensively.

Most typically, molly fish become fat and bloated due to pregnancy, in which they will also develop a gravid spot, spend more time hiding and become aggressive towards tankmates. However, mollies may also appear fat due to constipation, overfeeding, dropsy, bacterial and protozoal infections.

As we move forward, I will show you how to distinguish a pregnant molly fish from a sick one. In case your molly is indeed sick, I will present to you a few tips to make sure that the problem gets resolved as soon as possible.

Why Is My Molly So Fat? Is It Just Pregnancy?

Can fish get fat? Yes, they can.[1] Sometimes, it is intentional. That is to say, fish that live in environments with scarce resources will store fat deposits as a way of preparing for long periods during which food is absent.

But aquarium fish like mollies rarely experience such scarcity, especially if their owners are diligent. That begs the question: why are some mollies fat? Are they pregnant, or did they merely overeat? These are the questions many aquarists ask because they cannot help but compare humans to fish.

And in humans, overeating and pregnancy are the most prominent causes of obesity. This comparison isn’t wrong. Though, with mollies, identifying the exact cause of bloating isn’t so easy because there are far too many factors to consider, for instance:

1. Pregnancy

It isn’t wrong to suspect your mollies of being pregnant once they gain weight. Mollies are live-bearers. They give birth to live fry. If your tank has both male and female mollies, the chances that your female molly is pregnant are very high.

Mollies are also very easy to breed. They can retain sperm, using it to become pregnant later when the conditions in their vicinity suit them. That is in contrast to humans who don’t get to choose when to conceive.

In other words, even if all your mollies are female, pregnancy is still a viable cause where a fat molly is concerned. Don’t be so quick to dismiss the issue simply because your tank doesn’t contain any male mollies.

To determine whether your molly is pregnant or perhaps fat for other reasons, you should look for additional physical signs. The most prominent one would be a black line that develops on the lower part of their belly.[2]

When the pregnant molly is about to give birth, she will also present a typical behavior. You may notice that the fish swims at darker areas or behind plants and decorations. That could indicate that a pregnant molly is seeking a safe place to release its offspring.

2. Constipation

If you have ruled out pregnancy, then constipation should be your next consideration. Constipated mollies will produce either little or no waste. And when they produce waste, their feces are hard and trailing. Common causes of constipation include overfeeding, inadequate diets, and infections.[3]

Constipation can cause bloating in mollies. That is where the fish is so swollen that its swimming habits become erratic. The condition isn’t a mere inconvenience. Constipated mollies will stop eating altogether if their condition is allowed to progress. That could also lead to death.

3. Overfeeding

Overfeeding happens all the time. That is because, like most fish, mollies will keep eating if you keep feeding them. Overfeeding, on its own, can cause rapid weight gain in mollies. It is also associated with additional challenges.

For instance, it will increase the amount of waste produced by your mollies, not to mention the quantities of leftovers in the water. These elements will contribute to the concentration of ammonia and nitrites in the tank.[4]

If left unchecked, waste and leftovers will lower pH and oxygen levels, weakening the molly’s immune system and leaving it vulnerable to ailments such as fin rot. Along with clogging the filters and increasing the manifestation of mold, the deterioration of conditions in your tank can cause improper digestion.

That is likely to bring illnesses such as constipation and swim bladder disease. Since fish have complex bodies, every extra pellet you feed your molly matters. Their environments are even more delicate. It doesn’t take much to destroy the balance you have created in your aquarium.

Also Read: Molly Fish Swim Bladder Disease

4. Diseases

Some mollies are merely fat. If yours is bloated, to the point where the creature’s swimming capabilities have begun to suffer, you have to test for infections (viral, parasitic, bacterial, or protozoal).

Infections can lead to a build-up of fluids within a molly fish’s abdominal cavity, causing bloating.[5] Dropsy is the most common disease that aquarists associate with bloating. It is also one of the most frightening because it kills most fish that contract it.

In fact, it has become commonplace for aquarists to kill fish that develop dropsy because they want to spare the fish from additional suffering. Like many diseases that cause bloating, dropsy damages the internal organs.

Mollies with the ailment have lesions on their skin and scales that stand out. They are not merely fat. By looking at them, you can tell that their bellies are swollen. Dropsy can be caused by internal parasites and prolonged exposure to low temperatures.

How Can You Tell if Your Molly is Pregnant?

These signs indicate that your molly fish is pregnant:

  1. The molly will become slower and less active. 
  2. The fish will spend a lot more time hiding, particularly behind plants and decorations.
  3. Despite lethargy, a pregnant molly will develop aggressive tendencies, lashing out at any tankmates that enter its territory.
  4. Pregnant mollies develop a gravid spot, a triangular stain near the anal vent that will become larger and darker as the molly’s pregnancy progresses.
  5. If the molly is light in color, the fry’s eyes may appear in the creature’s belly, sometimes followed by black lines.

Mollies are live-bearers. When they are pregnant, their bellies swell. But how can you differentiate between a molly whose belly is swollen because it is pregnant and a molly whose belly is swollen as a result of one or more of the conditions mentioned above?

Mollies that are bloated because of conditions like constipation and dropsy have additional symptoms besides the swelling, such as loss of appetite, bulging eyes, and fading colors. However, a pregnant molly won’t show such concerning symptoms.[6]

How to Treat Fat and Bloated Mollies?

Mollies are not naturally fat. If you have failed to identify signs of pregnancy in a fat molly, you must act quickly to locate and then resolve the bloating causes before the fish’s condition worsens:

1. Dropsy

Many aquarists are quick to respond to bloating in mollies because they fear dropsy. They know that the condition kills most fish and wish to catch it early before lasting damage. That is a healthy attitude.

Though, if your molly is already bloated from dropsy, you are probably too late. Dropsy does irreversible damage to the internal organs, and if you can observe symptoms such as protruding scales, there is very little you can do to save the fish.[7]

But that doesn’t mean you should give up. You still have some treatment options you can try. First of all, you have to quarantine the sick fish. If a bacterial infection isn’t the cause of the disease, you can treat it with Epsom Salt (link to Amazon). Ensure that you add no more than 2.5 teaspoons for ten gallons of water in the hospital tank.[8]

Dropsy caused by bacterial infections cannot be treated, especially if the molly has already manifested symptoms. If you catch the disease early, your vet can prescribe some effective antibacterial medication.

2. Constipation

If your mollies have constipation, don’t feed them for two or three days. Perform a water change and raise the temperature by a few degrees. Once the molly’s fasting period ends, give it cooked, peeled peas, daphnia, chickweed, or other foods that can act as laxatives.

If the molly’s condition keeps worsening, pharmaceutical-grade Epsom salts can make a difference. Once the ailment clears, don’t be so quick to resume the molly’s previous diet. Prioritize whole foods that it can digest with ease. Give it small amounts.

3. Overfeeding

Try to avoid overfeeding. Feed the molly on a schedule. It shouldn’t eat more than twice a day. Add small amounts to the tank that it can eat in five minutes or less. Fight the urge to feed the molly only because it acts hungry. Fish will rush to the surface when they want you to feed them.

But you shouldn’t permit such behavior to deceive you. It would help if you also varied the molly’s diet. Include high-quality flakes and pellets, cucumber medallions, and zucchini medallions. If the fish is overweight, you can stop feeding it for a few days to control the situation. Make sure you remove all leftovers and waste.

4. Tank Conditions

Most ailments that mollies suffer can be attributed to the conditions in the tank. That includes bloating. Mollies require a pH of 7.5 to 8.2, temperatures ranging from 72 degrees F to 82 degrees, and a tank of at least ten gallons.[9]

Dirty water will introduce toxins such as ammonia, nitrates, and nitrites. These toxins can weaken your molly’s immune system, leaving it susceptible to diseases like dropsy that can cause bloating. To prevent this, I use the API Test Strips (link to Amazon) that measure the essential parameters within minutes.

Besides compromising the molly’s ability to breathe, inappropriate water conditions can also cause bloating in the absence of diseases like dropsy and constipation. You see this most commonly in cichlids that suffer from Malawi bloat, an ailment that develops when the tank’s conditions deteriorate. 

But mollies are also susceptible to a similar illness. That is why you have to perform regular water changes. Doing so will combat a variety of infections, not to mention eliminating toxins like chlorine. Start with weekly changes. Replace 10-15% of the water each time.

Eliminate all potential sources of stress, such as aggressive tankmates and overstocking. Check your filters, pumps, and heaters to ensure that they are functioning optimally. That will prevent drastic temperature changes and oxygen deficiencies.

Also Read: Molly Fish Swimming Vertically

5. Pregnancy

You can prevent unwanted pregnancies by removing all the males from your tank. If your molly is already pregnant, place it in a breeding box to protect it from the unwanted stress that community aquariums induce. Give it plenty of cover via plants and decorations.


The most common cause for a fat molly is pregnancy. Naturally, all pregnant mollies become fat since they are carrying living fry. If that is the case, you may notice the typical gravid spot or perhaps a black line at the belly’s bottom.

However, if your molly is a male or appears sluggish, it is probably sick. The first way of action should be isolation. That will prevent potential infections from spreading. Then, adjust the water in the hospital tank, so it matches the general molly’s requirements.


  1. https://thefisheriesblog.com/2018/01/08/fat-fish/
  2. https://www.aquariumnexus.com/pregnant-molly-fish/
  3. https://allnaturalpetcare.com/blog/2012/01/23/natural-prevention-treatment-constipation_bloat-freshwater-fish/
  4. https://www.petcoach.co/article/why-overfeeding-fish-is-a-problem-and-how-to-avoid-it/
  5. https://www.cuteness.com/article/treat-bloated-fish
  6. https://animals.mom.me/happens-fish-gets-big-stomach-sudden-8404.html
  7. https://www.thesprucepets.com/dropsy-in-aquarium-fish-1381806
  8. https://www.ratemyfishtank.com/blog/what-is-dropsy-and-how-do-i-treat-it
  9. https://aquariumtidings.com/molly-fish/